Process for preparing transition metal nitrides and transition metal carbonitrides and their reaction intermediates

ABSTRACT

A process for making ammonolytic precursors to nitride and carbonitride ceramics. Extreme reaction conditions are not required and the precursor is a powder-like substance that produces ceramics of improved purity and morphology upon pyrolysis.

The invention was developed pursuant to a contract with the UnitedStates Department of Energy.

This is a division of application Ser. No. 939,920, filed Dec. 9, 1986.

This invention is a process for preparing ceramic nitrides andcarbonitrides from ammonolytic intermediates that produce powder ceramicmaterials which are easy to use in manufacturing articles.

BACKGROUND OF THE INVENTION

Advanced ceramic materials are chemically inert compounds with highthermal stability and mechanical strength. Such characteristics makethese materials attractive candidates for applications such as heatengines, cutting tools, and turbine blades, articles which are presentlymade with expensive super alloys. Current interest in advanced ceramicscenters around such materials as carbides, nitrides, borides, andsilicides which have properties of hardness, corrosion resistance, andthermal stability that cannot be matched by metallic alloys or otherstructural materials. Examples of these ceramic materials are SiC, Si₃N₄, TiC, TiN, VC, WC, and BN. Other nitrides and carbonitrides areuseful as superconducting materials and include NbN, MoN, and Nb(C,N,).

Although chemical inertness of advanced ceramics is an advantage inthese applications, it makes fabrication of components through pressingand sintering a difficult task and places stringent demands on thepurity and morphology of the starting materials. Previously, thesecompounds were prepared by a very high temperature reaction in anitrogen atmosphere using metal oxide or pure metal powder and carbon asreactants. The reaction yielded clumps of product material that had tobe ground into a powder before it could be used. Not only are the hightemperature reaction and grinding steps difficult and costly processes,they can also be a serious source of contamination.

More recently, attempts have been made to make metal nitrides byreacting the transition metal halides with ammonia or a nitrogen andhydrogen gas mixture. The ammonia or nitrogen atmosphere not onlyprovides the reactant for making the nitride, but it also assures theabsence of oxygen which can cause damage of the final ceramic product ifit is present during the reaction. However, when titanium chloride wasreacted with ammonia at 1000° C., the titanium nitride product was inthe form of hard clumps that required grinding before they could beused, and there was also a hydrogen chloride by-product which isreactive and corrosive to the ceramic material.

Such problems have led researchers to attempt to develop appropriateprecursors that can be converted to ceramic materials by chemical meansrequiring less rigorous conditions and producing a product that is in amore readily usable form. This accomplishment would be an importantdevelopment in the area of ceramic production.

SUMMARY OF THE INVENTION

In view of the above needs, it is an object of this invention to providea process for making ceramics using readily available materials andmoderate reaction conditions.

It is also an object of this invention to provide a process for makingceramics that results in a product that is in a readily usable form anddoes not produce reactive by-products.

It is a further object of this invention to provide precursors for theprocess for making ceramics.

Another object of this invention is to provide a process for makingceramics that is fast and provides good yields.

It is also an object of this invention to provide ceramics that are pureand in fine particulate form. Upon further study of the specificationsand appended claims, further objects and advantages of this inventionwill become apparent to those skilled in the art.

To achieve the foregoing and other objects in accordance with thepurpose of the present invention, the process of this invention maycomprise a process for making an organometallic precursor to ceramicswherein a transition metal halide is mixed with a borohydride in liquidammonia solvent. A solid compound of a transition metal haloamide isformed which can be easily separated from the soluble ammonium halideand ammonium borohydride by-products. This solid compound is then mixedwith a salt that will react with the transition metal halide bydisplacing the halogen atom and forming a halide salt. This mixing isdone in liquid ammonia solvent to produce a complex compound oftransition metal amide, the precursor to the transition metal nitride.If the precursor to the nitride is desired, an amide salt is used, andif the precursor to the carbonitride is desired, a carbon salt is used.The invention is a precursor that is an organometallic amide of thetransition metal. The invention is also a process for making ceramicsand comprises pyrolyzing the precursor to produce the nitride orcarbonitride.

Among the many advantages of forming ceramics in this way includes areaction that proceeds at moderate conditions, formation of fineparticulate precursors of high metal content resulting in minimumshrinkage upon pyrolysis, and a very pure ceramic with controlled carboncontent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preparation of advanced ceramic materials requires, in general, theexclusion of oxygen since the presence of this element can havedetrimental effects on the properties of some ceramics. Thus, the use ofoxygen-free reactants and solvents is almost a necessity in thepreparation of precursors. For this reason, liquid ammonia is a usefulreaction medium for conducting this type of synthetic chemistry.Titanium halides were chosen as convenient starting materials since theyare readily available and exhibit a high degree of purity. Previous workhas been done on titanium chloride at high temperatures of about 1000°C. with ammonia. The reaction required about two days and resulted in atitanium nitride solid mass of material in the form of clumps that wasdifficult to handle without further processing. In addition to theunfavorable morphology of the ceramic material, the reaction alsoproduced a reactive and corrosive by-product hydrogen chloride.

At room temperature, titanium IV underwent ammonolytic reactions inliquid ammonia to yield essentially TiX(NH₂)₃.2NH₃ ; however, thisreaction required several days due to a competing reverse reaction andproduced an ammonium halide by-product which was not easily separatedfrom the titanium haloamide. In attempts to promote the reaction andprevent a reverse reaction, alkali metal borohydride was added to reactwith the ammonium halide byproduct. The products of this side reaction,an alkali halide salt and aminoborohydride, were soluble in liquidammonia and easily washed away from the solid primary product, atransition metal haloamide. The success of using the borohydride toblock the reverse reaction was the first step in the development of theprocess of this invention.

The transition metal haloamides were treated with a carbon salt or anamide salt depending on whether the carbonitride or the pure nitride wasdesired. The carbon salt must be one that will attach to the transitionmetal and remove the halogen by formation of a halide salt. A suitablecarbon salt is sodium acetylide which produces a transition metalacetylide derivative. Such compounds comprise the precursor to theceramic carbonitride.

An advantage to forming ceramics using this process is that the carboncontent of the carbonitride can be controlled since the carbon of thecarbon salt replaces the halogen of the transition metal haloamide;therefore, the greater the number of halogen molecules in the transitionmetal halide, the higher the carbon content of the final ceramic willbe.

To be more specific, the precursors are made by starting with a halideof a transition metal such as vanadium, tungsten, molybdeunum, titanium,or niobium which is mixed with an alkali metal borohydride such aspotassium borohydride or sodium borohydride, or other suitableborohydrides, in liquid ammonia at room temperature. The reaction,ammonolysis, is a parallel reaction to hydrolysis

    MX.sub.n +2xNH.sub.3 →MX.sub.n-x (NH.sub.2).sub.x +xNH.sub.4 X

where M is a transition metal, X is a halogen, and n and x arevariables. The side reaction of a borohydride of an alkali metal, suchas potassium, with the ammonium halide by-product prevents a reversereaction.

    NH.sub.4 X+KBH.sub.4 →KX+NH.sub.4 BH.sub.4

    NH.sub.4 BH.sub.4 →NH.sub.3 BH.sub.3 →H.sub.2 ↑

These by-products are soluble in liquid ammonia and easily washed away.

The insoluble metal haloamide is then reacted with a carbon salt of ametal that will attach to the halide of the metal haloamide producing aninsoluble organometallic amide and soluble metal halide salt. Sodiumacetylide is a good choice for the carbon salt.

    nMX.sub.n-x (NH.sub.2).sub.x +NaC.tbd.CH→M.sub.n (C.tbd.CH).sub.n-x (NH)+xNaX

Since the number of carbon atoms is in direct relation to the number ofhalogens of the metal haloamide, the carbon content of the carbonitridecan be controlled. For example, one can begin with either TiBr₄ to formTiBr(NH₂)₃ or TiBr₃ to form TiBr₂ (NH₂). TiBr(NH₂)₃ reacts withNaC.tbd.CH to yield a Ti₄ (C.tbd.CH)(NH)₆ NH₂ precursor, whereas TiBr₂(NH₂) reacts with NaC.tbd.CH to yield Ti₄ (C.tbd.C)₂ (NH)₄, a precursorwith twice the carbon content as the preceding one. Generically, theorganometallic amide can be characterized a M^(n) (C.tbd.CH)_(x)(NH₂)_(n-x) where n is the oxidation state of the metal and x is avariable. The resulting organometallic amide is the precursor to thefinal ceramic which is obtained by pyrolyzing the precursor at about800° C. under a dynamic vacuum to give a fine pure powder that is inreadily usable form.

If the nitride is desired instead of the carbonitride, then the metalhaloamide is reacted with an amide salt of a metal that will attach tothe halide. Depending on which halogen comprises the metal haloamide,the suggested salt can be either potassium amide or sodium amide, thepotassium being suitable for bonding to a bromine and sodium suitablefor chlorine. For example, to make the titanium nitride, a titaniumhaloamide such as TiBr(NH₂)₃ is reacted with KNH₂ to produce KTi(NH₂)₂which upon pyrolysis gives the powdered ceramic TiN product.

EXAMPLE

A heavy walled ampoule was loaded with 2.17 mmole of NbBr₅ and 11.8mmole KBH₄. Eleven grams of ammonia was condensed in the ampoule whichwas then sealed. After standing overnight at room temperature, theampoule was opened and it was found that a reaction had taken placeproducing 4.24 mmole H₂ /mmole of Nb and a brown precipitate, theammonolytic product, containing 70% of the initial niobium. Theprecipitate had a composition corresponding to NbBr(NH₂)₂ NH. When thismaterial was treated with NaC.tbd.CH in liquid ammonia, it produced aderivative having a composition corresponding to Na₄ Nb₆ (C.tbd.C)₄ (N)₈NH. Pyrolysis of the derivative at 800° C. under vacuum resulted in a30% weight loss and left a residue identified by its x-ray diffractionpattern as niobium carbonitride. Chemical analysis of this materialrevealed the composition to be 88.4% Nb, 4.4% C, 6.1% N, 0.1% Br and1.0% Na.

I claim:
 1. A process for making a precursor to ceramicscomprising:placing a transition metal halide and an alkali metalborohydride in liquid ammonia solvent resulting in the formation of aninsoluble transition metal haloamide and soluble by-products, alkalimetal halide and ammonium borohydride; separating said insolubletransition metal haloamide from said solvent and said solubleby-products; reacting said transition metal haloamide with a salt inliquid ammonia, said salt being one that will react by displacing thehalogen atom of said transition metal haloamide and yield a precursor toceramics.
 2. The process of claim 1 wherein said salt is a carbon saltand said precursor is an organometallic amide.
 3. The process of claim 1wherein said salt is an amide salt and said precursor is a transitionmetal amide.
 4. The process of claim 1 wherein said transition metal isselected from the group vanadium, tungsten, molybdenum, titanium,niobium;said halide is selected from the group chloride, bromide, andiodide; and said alkali metal borohydride is selected from the grouppotassium borohydride and sodium borohydride.
 5. The process of claim 4wherein said salt is a carbon salt and said precursor is anorganometallic amide.
 6. The process of claim 4 wherein said salt is anamine salt and said precursor is a transition metal amine.